Impeller cut and speed association...

[Scarab Jet]

I am trying to figure out why it is that [let's say] a "C" impeller in a given boat gives it a higher top-end speed than [let's say] an "A" impeller... Is it because the C cut basically moves less water quicker vs. the A cut that moves more water but slower?
Or do I have the whole reasoning wrong? If so, I appreciate if someone can explain the physics behind what makes a particular cut impeller provide better top-end but slower hole-shot vs. the one that provides exactly the opposite...
Thanx in advance...

[SmokinLowriderSS]

I can't even touch this, but I might just learn somethin'. :idea:
Someone from the highly experienced "old guard" who have been running and tuning jets for decades I am sure will come and educate both of us. I have been told the same thing, by DuaneHTP, but didn't ask for an explanation at the time. I'm sure a lot depends of setup. If an engine is being held below it's best power, an impeller cut will make it faster as the engine can now wind up to it's power, allowing more pump pressure to build. Beyond that obviousness, I'm gonna sit back & get a lesson.

[Duane HTP]

Scarab, check your PM.

[flat broke]

That's a good question Scarab Jet, and while I'm sure Duane has you covered in the PM, it might not be a bad idea to address the topic here on the forum for others to learn from.
Simply put, think of the sizes of impellers refer to how much water they will move in each revolution. An AA takes a bigger gulp of water per revolution than a C. The total amount of thrust, which is the volume of water moved per revolution multiplied by the revolutions per minute of the impeller, dictates how the boat will perform. Physics are blind to the make, model, and size of our boats and components. It always boils down to applied force vs. the various resistances we are trying to overcome.
If we give each impeller a numerical value for units of water per revolution this will start to make more sense. For the sake of illustration refer to the following examples:
D cut - 1 unit
C cut - 2 units
B cut - 3 units
A cut - 4 units
AA cut - 5 units
If your motor could turn each of the impellers to 5000RPM, you would see between 5000 and 25,000 units per minute moved through the pump. It wouldn't take much to decide that you might as well move 25,000 units of thrust for the same cost as 5000. Unfortunately with each increase in units of water/revolution there is a corresponding increase in the required HP to turn the impeller. So let’s say that the HP requirements at 5000RPM for each impeller are as follows(These are not accurate numbers, but exaggerations to illustrate a point)
D to 5k - 100hp
C to 5k - 200hp
B to 5k - 300hp
A to 5k - 400hp
AA to 5k -500hp
If the motor in your Scarab makes 300hp at 5kRPM, then you would likely choose a B impeller, as it will place the motor operating at its peak HP when at WOT. Pretty simple right? Well, what if your motor made it's 300hp at 550RPM? Now we have a bit of a problem. In order to get to the peak HP, we need to spin more RPM than the B impeller will allow us to. A compromise is in order. We need to balance spinning a less efficient impeller (a C cut) faster to get to the peak HP of the motor; against the efficiency losses associated with going from a B impeller to a C. Because this is a fictitious example, we are in luck and can easily figure out what we should do. Assuming we'd be able to spin a C to 5500RPM with your motor to get to your peak HP, we know that we would be able to move 11,000 units of water per minute (2 units per revolution X 5500RPM). If you were only able to turn the B impeller to say 4500RPM with the same motor, you would be moving 13,500 units of water at 4500RPM. In this instance, even though you're not able to get to your peak HP which is at 5500RPM, the efficiency of the B impeller at a lower RPM still moves more water than the C impeller at a higher RPM. In a real world scenario, it would likely be the case that the motor in question made little more HP between 4500RPM and 5500RPM.
With that example under our belts, let’s look at your original question, "I am trying to figure out why it is that [let's say] a "C" impeller in a given boat gives it a higher top-end speed than [let's say] an "A" impeller... Is it because the C cut basically moves less water quicker vs. the A cut that moves more water but slower?
From the example above we can see that in each boat with different peak HP RPMs and HP levels, it is not always a given that a smaller impeller will make a boat move faster. In actuality the C moves an equal amount of water slower than the A impeller; as it will require more revolutions of the C impeller to move the same amount of water as the A impeller will move in one revolution.
Your question touches on what I think is the biggest gap in our knowledge about jet drives and the efficiency associated with each impeller size. If there were accurate published information saying that at a given RPM and inlet pressure, each impeller moved X GPH/GPM/whatever, we would be able to much more accurately determine which impeller would create the optimum efficiency for a particular boat. Without that information, we rely on matching the impeller to the peak power of the engine to get the most out of the equation. However, just like in our example, there are real world scenarios where a bigger impeller at lower RPM will go faster and accelerate harder than a smaller impeller at a higher RPM. Just this last weekend I was out on a boat that runs 2 mph faster with a AA Impeller compared to an A impeller. The boat looses 2-300RPM by going to the larger impeller, but the total amount of water moved as a result of using the bigger impeller is higher. This results in more applied force against the various resistances the boat sees.
While the above was a very general oversimplification, not accounting for many other factors such as running gear setup, internal losses in the pump etc. It illustrates the fact that the smaller impeller isn't always faster. It's the impeller that can move the most water in a given period of time with the available horsepower, which will ultimately be the most efficient setup.
Chris

[MikeF]

Some more insight to the question you pose is in the Feb 2005 issue of HB mag. Cyclone did ALOT of testing that day for an article for HB.
:cool:

[flat broke]

Some more insight to the question you pose is in the Feb 2005 issue of HB mag. Cyclone did ALOT of testing that day for an article for HB.
:cool:
Mike, were you refering to me, or to Scarab Jet? The data I'd like to see is nowhere in that article. I doubt any jet mfg is going to spend the $$$, (and it would be lots of $$$) to do a complete test to determine the actual flow rates of their impellers at a given RPM. There are probably folks out there that have a strong enough handle on fluid dynamics to extrapolate some flow numbers based upon the physical dimensions of the impeller, losses, through the pump etc. But unless they have a chubby for jetboats, I doubt we'll ever see much published on the subject.
Which gets me to thinking. I know at Lake Powell, they have a sample of one of the turbine impellers out on display. It looks like a mixed flow design and it's huge. Do you think they have done the research to know how many gph or whatever they measure in is moved with each revolution at a given RPM? In a sense, I would almost think they have to know. If they need to spin the Generator x RPM to make X kilowatts, they have to know how much water to release right?
Just looking for a place to find information where someone might have had deep enough pockets and justification to do the research.
Chris

[UBFJ #454]

Talk with Don Bowers ... He did a lot of flow testing when designing his inducer and has a lot of information dealing with impellers and their flow rates at maximum pump throughput.

[MikeF]

Mike, were you refering to me, or to Scarab Jet?
SJet and anyone who wanted to look at the article.
Which gets me to thinking. I know at Lake Powell, they have a sample of one of the turbine impellers out on display. It looks like a mixed flow design and it's huge. Do you think they have done the research to know how many gph or whatever they measure in is moved with each revolution at a given RPM? In a sense, I would almost think they have to know. If they need to spin the Generator x RPM to make X kilowatts, they have to know how much water to release right?
Chris
I'm sure they did the research. :boxed: So much so they make $$$$$ with theirs.:D

[Scarab Jet]

Thanx Chris... I really appreciate such a comprehensive explanation on this subject that we all jetters can benefit from...
Your thorough explanation brings up a couple of more questions that I appreciate your response to [in advance]:
1) So basically the long standing myth that "by going to a higher cut impeller, one would automatically gain a better hole shot and loose some speed on the top end at the same time" is not necessarily always true, correct?
and
2) In your illustration, you mention the most horsepower that an engine makes in a given RPM is the number to shoot for when comparing one impeller cut vs. another... But, how about maximum torque for a given RPM that a given engine makes? I.e., shouldn't we also look at that number when selecting an impeller? Or should we just be concerned about HP number alone? Or perhapos a combination of both (somehow)?
Thanx again in advance...
Mike...

[Konabud]

I not the expert but I think the main thing is to load your engine properly at that engines max RPM (not unlike a boat propeller) Boat weight and several other factors might come into play but at the end of the day you want your engine to turn Max RPM at WOT

[LVjetboy]

Scarab,
Both speed and acceleration depend on how much engine power you put to the impeller and how well that power is converted to thrust as Chris posted. Regardless of boat design or application, maximum thrust = best performance no matter what mix of acceleration or top end is important to you.
That's why you hear about impeller cut matched to your engine's power curve. How well that power is converted to thrust during launch and speed is why you hear about hole shot and intake loading. Compromises. Boat design and application are where efficiency compromises kick in.
For example a B cut matched to peak engine power may deliver maximum thrust with a fully loaded intake at speed, but suck air on launch and cavitate during acceleration...a dog. Cavitation means less power converted to thrust.
Heavier boats are more likely to cavitate a small impeller over a wider speed range because they spend more time at low speed with less than ideal intake loading. BTW, cavitation can be localized (not full) not easily identified and still lead to losses. And flow losses from cut size may also come from blade-to-vane spacing. A smaller impeller cut releases water sooner from the blade trailing edge before it enters the bowl vane leading edge. This could cause added turbulant flow loss between the airfoils. And spining those blades faster (with a smaller cut size) means higher angle of attack and more likely partial cavitation.
Does that mean applying maximum power to the impeller is bogus and instead torque is the driver or even more of a stretch, some unknown combination of power and torque? Nope. Just means for a specific boat design, weight and application, impeller loading issues or how well power is converted to thrust can overshadow power applied.
No one understands every detail of physics or mechanics for our jet pumps. And there's no automatic answer for best cut size. But some general principles and basic laws of physics that ARE known to help guide us. One of those principles is that power applied to the impeller (not torque) drives performance. That's because power includes rpm. Balancing that is how effectively that power is converted to forward thrust. This is where losses from impeller cavitation, flow bending and blockage, etc are important and offset gains from more power applied.
MikeF mentioned the HBM article on impeller size testing. If I remember the conclusion was bigger is better. I questioned some results but that's history. Here's another test...
Impeller RPM Speed
American Turbine:
AT 9.5 5200 89.3
Aggressor:
AA 5400 92.4
A 5900 101.8
B 6300 94.6
C 6600 91.2
900 hp engine, jet drive and performance hull. As for top speed, these tests show bigger is not always better to the tune of a loss in 12+ mph.
jer

[LVjetboy]

Bear good point. I'd like to know how much an inducer can offset any launch/acceleration efficiency hit for a smaller impeller matched to peak power top end.
jer

[Ken F]

Hey Jer good to see you back!
Ken F

[flat broke]

Mike (Scarab Jet),
I think Jer summed up both your questions more succinctly than I would have been able to. But to further answer the tq vs hp question; there are correlations between how motors build tq and hp. So if we look at the hp, we by definition get the tq value (one is derived from the other). For this reason, it is better to use the power rating because it takes into account the function of RPM as Jer stated.
Jer and Bear,
On the inducer question, There have to be some before and after scenarios from Jack, Duane, or someone who has done this. I think Cyclone might be running an inducer in his pump now? I know he had to pedal the hell out of the boat to keep it from cavitating when he was first setting everything up. Perhaps he can share his experiences. I'd love nothing more than to hear that the inducer would allow a "B" to leave like a "AA", but I have a feeling that may be a little over optimistic :D

[sleekcrafter]

Some boats just won't launch from a dead stop, the pump pumps all the water from beneath the boat in a sense. Some must rely on a launch controller, to keep cavitaion at bay. Several factors come into play, Horse power, Hull type, Weight, Loader design, and Impeller. Some can flat foot from a stand still, others may not.
Sleek

[LVjetboy]

"I'd love nothing more than to hear that the inducer would allow a "B" to leave like a "AA", but I have a feeling that may be a little over optimistic"
Ya me too. If it did I'd break my transom seal to put one in. Back-to-back impeller testing is tough to do accurately (do it right) the same I'm sure for an inducer. And if the gain's hard to measure, there's less incentive to do a test. Still, I'd love to see a test of this product for an AA to B impeller cut and a healthy (dyno tested) engine, let's say with a heavy jet to stack the deck.
jer

[Cas]

This was on another board not too long ago. I DID NOT WRITE IT!
This is only my opinion, and I am sure there are some who will argue with my thinking, but if you do the math, and use common horse sense, it is pretty easy to see it is really just a way for a guy to stuff YOUR money into his pocket. And that is why they call it a stuffer, in my opinion.
It is supposed to close up the gap between the back of the impeller and front of the lip inside the bowl and keep water from touching the back side of the impeller. But after building pumps for over 12+ years, the worst gap I have ever run across was .220" and that is a little less than 1/4 inch. And from the factory, Berkeley in this case, they normally run between .140-190" with a straight wear ring. Now, if a guy was going to blueprint a pump the old school way, he would cut the shaft to move the impeller closer to the suction housing to make the front of the impeller seal up tighter to the suction/wear ring face to keep water from cavitating, or recirculating past the side of the wear ring and the impeller, and that would increase the gap on the back side by about .125". Which if totaled up, would reach about a .300" gap. That seems like quite a bit of a gap, and that is the reason they were invented. But you have to look at which way the water is moving through the pump to see if the water could even make a 90 degree turn to get between the bowl and back of the impeller, even with a .300" gap. Water takes the path of least resistence.
And if you take the speed of the water moving through the pump into consideration, with the speed the boat is moving, it is clear to see that at slow speed, yes, water may be able to get behind the impeller. But if going 60 mph, then the water is moving so fast, it can not make the turn and will go right into the bowl, without even trying to go behind the impeller with a .300" gap.
Now if you use a, new style, shouldered wear ring, and it is plain dumb not to when you rebuild your pump or have it built, this closes up the gap both in the front of the impeller and the gap in the back between the bowl also. And it is even harder at slow speed for the water to make any turn behind the impeller. Ideally, you want a .030" gap in both front and rear, on a non-race pump.
There are still some builders who use the straight wear rings, but now the shouldered types are offered by most kit manufacturers, and they come in stainless(good), bronze(better), and Duane makes one that is Urethane, which is the best for a race pump, in my opinion, so why put 30 year old parts into a pump you are building today?
And some may argue, that the water drag at high speed is causing drag on the back of the impeller, but it is just water that is not moving and is of no consequence to create enough drag on an impeller to even record.
Much like the plate inside the droop snoot was supposed to create some additional lift, but it never added any more than a droop without a plate. So they call it a water straightener instead. And it doesn't make any difference either, from testing with all the styles available. Go figure.

[El Prosecutor]

I think the above quote is good info but is talking about a stuffer plate as opposed to an inducer.

[cyclone]

Chris- i am running an inducer now. I wasn't able to isolate it as the sole fix for my setup though because Tom Papp made a couple other changes to my pump at the same time he installed the inducer. I've left it in the pump because i do believe it helps in my case (blown motor and AA impeller) and it doesn't seem to have affected the top end speed.
Before the pump mods i could only give the engine part throttle out of the hole before the pump would cavitate. After the mods I could nearly flat foot the gas without the pump cavitating.
He was able to take three tenths of a second off my half track times so needless to say i am pleased.

[djdtpr]

Heres where i get confused,My boat is overloading at the half track and blowing the tail i was told maybe we will cut the impeller down a little more to move more water through there.How will a BC move more than a AB or B?

[garret]

Heres where i get confused,My boat is overloading at the half track and blowing the tail i was told maybe we will cut the impeller down a little more to move more water through there.How will a BC move more than a AB or B?
well why dont you go answer your own question with all this extra time you have

[flat broke]

Danny,
Assuming both impellers are stock, a smaller cut will not move more water than a bigger one at the same RPM. Now if you have a smaller detailed impeller, it could be that the countour of the blade has been changed to actually create more surface area, even though the blade is shorter from front to back than one on a larger impeller.
When you say overloading, I assume you mean the intake. Get Mike's quickdata on there and check your bowl pressures compared to your intake pressure. It's possible that what Tom means is that the smaller impeller wont stack as much pressure, so it won't ass lift, and hence your times might be quicker. But tit for tat, the smaller impeller shouldn't move more water at the same RPM. Before you swap impellers, try opening up the nozzle I.D. on the end of the pump. Reducing restriction there could lower your bowl pressure and reduce some of the overcharge in the intake. Then of course there is always your shoe depth and loader design to look at as well.
I'm not second guessing Tom, just throwing out what I'd look at if it was my boat and I was having the problem.
Chris